An urban forest is a forest or a collection of trees that grow within a city, town or a suburb. In a wider sense it may include any kind of woody plant vegetation growing in and around human settlements. In a narrower sense (also called forest park) it describes areas whose ecosystems are inherited from wilderness leftovers or remnants. Care and management of urban forests is called urban forestry. Urban forests may be publicly-owned municipal forests, but the latter may also be located outside of the town or city to which they belong.

Urban forests play an important role in ecology of human habitats in many ways: they filter air, water, sunlight, provide shelter to animals and recreational area for people. They moderate local climate, slowing wind and stormwater, and shading homes and businesses to conserve energy. They are critical in cooling the urban heat island effect, thus potentially reducing the number of unhealthful ozone days that plague major cities in peak summer months.

In many countries there is a growing understanding of the importance of the natural ecology in urban forests. There are numerous projects underway aimed at restoration and preservation of ecosystems, ranging from simple elimination of leaf-raking and elimination of invasive plants to full-blown reintroduction of original species and riparian ecosystems.

In Adelaide, South Australia (a city of 1.3 million), Premier Mike Rann (2002 to 2011) launched a major urban forest initiative in 2003 to plant 3 million native trees and shrubs by 2014 on 300 project sites across the metro area. The projects range from large habitat restoration projects to small amenity gardens and local biodiversity projects. Thousands of Adelaide citizens have participated on well publicised community planting days. Sites include parks, reserves, transport corridors, schools, water courses, coastline council land and other public open space. Only indigenous trees and shrubs native to the particular local area are planted to ensure genetic integrity. Premier Rann said the project aimed to beautify and cool the city and make it more liveable; improve air and water quality and reduce Adelaide's greenhouse gas emissions by 600,000 tonnes of C02 a year. He said it was also about creating and conserving habitat for precious wildlife and preventing species loss.[1]

The benefits of urban trees and shrubs are many, including beautification, reduction of the urban heat island effect, reduction of stormwater runoff, reduction of air pollution, reduction of energy costs through increased shade over buildings, enhancement of property values, improved wildlife habitat, and mitigation of overall urban environmental impact.[5]

The presence of trees reduces stress, and trees have long been seen to benefit the health of urban dwellers.[6] The shade of trees and other urban green spaces make place for people to meet and socialize and play. The Biophilia hypothesis argues that people are instinctively drawn to nature, while Attention Restoration Theory goes on to demonstrate tangible improvements in medical, academic and other outcomes, from access to nature. Proper planning and community involvement are important for the positive results to be realized.

Trees and shrubs provide nesting sites and food for birds and other animals. People appreciate watching, feeding, photographing, and painting urban wildlife and the environment they live in. Urban trees, shrubs and wildlife help people maintain their connection with nature.

The economic benefits of trees and various other plants have been understood for a long time. Recently, more of these benefits are becoming quantified. Quantification of the economic benefits of trees helps justify public and private expenditures to maintain them. One of the most obvious examples of economic utility is the example of the deciduous tree planted on the south and west of a building (in the Northern Hemisphere), or north and east (in the Southern Hemisphere). The shade shelters and cools the building during the summer, but allows the sun to warm it in the winter after the leaves fall.

As cities struggle to comply with air quality standards, the ways that trees can help to clean the air should not be overlooked. The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides (NOx), sulfuric oxides (SOx) and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds (VOCs) in the presence of sunlight. High temperatures increase the rate of this reaction. Vehicle emissions (especially diesel), and emissions from industrial facilities are the major sources of NOx. Vehicle emissions, industrial emissions, gasoline vapors, chemical solvents, trees and other plants are the major sources of VOCs. Particulate pollution, or particulate matter (PM10 and PM25), is made up of microscopic solids or liquid droplets that can be inhaled and retained in lung tissue causing serious health problems. Most particulate pollution begins as smoke or diesel soot and can cause serious health risk to people with heart and lung diseases and irritation to healthy citizens. Trees are an important, cost-effective solution to reducing pollution and improving air quality.

Trees reduce temperatures and smog

With an extensive and healthy urban forest air quality can be drastically improved. Trees help to lower air temperatures and the urban heat island effect in urban areas (see: 'Trees are energy savers' for more information on this process). This reduction of temperature not only lowers energy use, it also improves air quality, as the formation of ozone is dependent on temperature.

In Sacramento County, California, it was estimated that doubling the canopy cover to five million trees would reduce summer temperatures by 3 degrees[vague]. This reduction in temperature would reduce peak ozone levels by as much as 7% and smoggy days by 50%.

Temperature reduction from shade trees in parking lots lowers the amount of evaporative emissions from parked cars. Unshaded parking lots can be viewed as miniature heat islands, where temperatures can be even higher than surrounding areas. Tree canopies will reduce air temperatures significantly. Although the bulk of hydrocarbon emissions come from tailpipe exhaust, 16% of hydrocarbon emissions are from evaporative emissions that occur when the fuel delivery systems of parked vehicles are heated. These evaporative emissions and the exhaust emissions of the first few minutes of engine operation are sensitive to local microclimate. If cars are shaded in parking lots, evaporative emissions from fuel and volatilized plastics will be greatly reduced.

Cars parked in parking lots with 50% canopy cover emit 8% less through evaporative emissions than cars parked in parking lots with only 8% canopy cover.

Due to the positive effects trees have on reducing temperatures and evaporative emissions in parking lots, cities like Davis, California, have established parking lot ordinances that mandate 50% canopy cover over paved areas.

"Cold Start" emissions

The volatile components of asphalt pavement evaporate more slowly in shaded parking lots and streets. The shade not only reduces emissions, but reduces shrinking and cracking so that maintenance intervals can be lengthened. Less maintenance means less hot asphalt (fumes) and less heavy equipment (exhaust). The same principle applies to asphalt-based roofing.

Active pollutant removal

Trees also reduce pollution by actively removing it from the atmosphere. Leaf stomata, the pores on the leaf surface, take in polluting gases which are then absorbed by water inside the leaf. Some species of trees are more susceptible to the uptake of pollution, which can negatively affect plant growth. Ideally, trees should be selected that take in higher quantities of polluting gases and are resistant to the negative affects they can cause.

Urban forest managers are sometimes interested in the amount of carbon removed from the air and stored in their forest as wood in relation to the amount of carbon dioxide released into the atmosphere while running tree maintenance equipment powered by fossil fuels.

Interception of particulate matter

In addition to the uptake of harmful gases, trees also act as filters intercepting airborne particles and reducing the amount of harmful particulate matter. The particles are captured by the surface area of the tree and its foliage. These particles temporarily rest on the surface of the tree, as they can be washed off by rainwater, blown off by high winds, or fall to the ground with a dropped leaf. Although trees are only a temporary host to particulate matter, if they did not exist, the temporarily housed particulate matter would remain airborne and harmful to humans. Increased tree cover will increase the amount of particulate matter intercepted from the air.

Large evergreen trees with dense foliage collect the most particulate matter.

The Chicago study determined that trees removed approximately 234 tonnes of particulate matter less than 10 micrometres (PM10) in 1991.

Large healthy trees greater than 75 cm in trunk diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 10 cm in diameter (0.02 kg/yr).

One important thing to consider when assessing the urban forest's effect on air quality is that trees emit some biogenic volatile organic compounds (BVOCs). These are the chemicals (primarily isoprene and monoterpenes) that make up the essential oils, resins, and other organic compounds that plants use to attract pollinators and repel predators. As mentioned above, VOCs react with nitrogen oxides (NOx) to form ozone. BVOCs account for less than 10% of the total amount of BVOCs emitted in urban areas. This means that BVOC emissions from trees can contribute to the formation of ozone. Although their contribution may be small compared with other sources, BVOC emissions could exacerbate a smog problem.

Not all species of trees, however, emit high quantities of BVOCs. The tree species with the highest isoprene emission rates should be planted with caution:

Trees that are well adapted to and thrive in certain environments should not be replaced just because they may be high BVOC emitters. The amount of emissions spent on maintaining a tree that may emit low amounts of BVOCs, but is not well suited to an area, could be considerable and outweigh any possible benefits of low BVOC emission rates.

Trees should not be labeled as polluters because their total benefits on air quality and emissions reduction far outweigh the possible consequences of BVOC emissions on ozone concentrations. Emission of BVOCs increase exponentially with temperature. Therefore, higher emissions will occur at higher temperatures. In desert climates, locally native trees adapted to drought conditions emit significantly less BVOCs than plants native to wet regions. As discussed above, the formation of ozone is also temperature dependent. Thus, the best way to slow the production of ozone and emission of BVOCs is to reduce urban temperatures and the effect of the urban heat island. As suggested earlier, the most effective way to lower temperatures is with an increased canopy cover.

These effects of the urban forest on ozone production have only recently been discovered by the scientific community, so extensive and conclusive research has not yet been conducted. There have been some studies quantifying the effect of BVOC emissions on the formation of ozone, but none have conclusively measured the effect of the urban forest. Important questions remain unanswered. For instance, it is unknown if there are enough chemical reactions between BVOC emissions and NOx to produce harmful amounts of ozone in urban environments. It is therefore, important for cities to be aware that this research is still continuing and conclusions should not be drawn before proper evidence has been collected. New research may resolve these issues.